Nuclear reactors cooled by water, and particularly pressurized-water nuclear reactors, include assemblies consisting of a bundle of very long fuel rods arranged parallel to one another and held in a framework formed by guide thimbles, spacers, and two end nozzles. The guide thimbles are arranged in the longitudinal direction of the assembly and are connected to transverse spacers spaced evenly along the length of the assembly.
The guide thimbles are also connected at each of their ends to one of two nozzles, constituting components for stiffening and closing the assembly.
The fuel rods of the assembly constitute a bundle in which the rods are parallel to each other and arranged in the transverse sections of the assembly, in a uniform pattern defined by the spacers. Some positions of the bundle are occupied by guide thimbles which are generally connected rigidly to the spacers.
The guide thimbles are longer than the fuel rods and are placed in the bundle so as to include a part which projects relative to the bundle of fuel rods at each of their ends. The nozzles are fixed to these projecting end parts of the guide thimbles so as to close the assembly at each of its ends.
The fuel rods consist of sintered pellets of nuclear fuel substance stacked up inside metal cladding isolating the pellets from the fluid surrounding the fuel assembly. In the event of the cladding of a rod of a fuel assembly breaking, it is necessary to replace this rod very rapidly in order to avoid leaks of radioactive product into the fluid for cooling the reactor. In order to gain access to the fuel rods and replace them, it is necessary to dismount one of the nozzles of the assembly, which involves eliminating the links between the end parts of the guide thimbles and the nozzle.
The nozzles include a transverse plate, termed adaptor plate, including through-holes reproducing the pattern of the guide thimbles in each of which a guide thimble is engaged and fixed.
To replace the defective rods in fuel assemblies, novel fuel assemblies including guide thimbles have been designed and developed in which the link with at least one of the end nozzles is dismountable.
In order to replace the defective fuel rods, the assembly is placed under water in a vertical position, in a pit such as a storage pit, such that the assembly rests on the bottom of the pit via one of its nozzles, or bottom nozzle. The other nozzle, or top nozzle, is accessible under a certain depth of water from the top of the pit.
In one type of known dismountable fuel asembly, those parts of the guide thimbles which are engaged in the top nozzle of the assembly include a radially expansible part, which may, for example, be a split bush connected to the end of the guide thimble. An immobilizing sleeve inserted inside the guide thimble radially expands the split bush and fastens the guide thimble, a radially projecting part of which becomes accommodated inside a cavity machined in the nozzle.
Such a device requires special forming of the end of the guide thimble which is to be engaged inside the adaptor plate of the nozzle.
Furthermore, the fitting and extraction of the sleeves for immobilizing the guide thimbles when assembling or dismounting the fuel assembly require special tools allowing tension or thrust to be exerted on the immobilizing sleeve and allowing the immobilizing sleeves to be dismounted and removed, leading to the formation of radioactive waste.
Fuel assemblies have also been proposed which include linking means which can be maneuvered quickly by turning an immobilizing element through a fraction of a turn, in order to lock or unlock the link between a tie for holding the assembly and the dismountable nozzle.
Such devices are described, for example, in U.S. Pat. No. 4,064,004 and U.S. Pat. No. 5,268,948, and have a complicated structure and many components.
Furthermore, these devices require the guide thimble to project above the adaptor plate of the nozzle and include parts projecting above the adaptor plate which are fixed to the end of the guide thimble and have an appreciable height which is incompatible with the use of consumable poison clusters. These parts include a bearing plate coming into a superposed arrangement relative to the adaptor plate of the fuel assembly, when the consumable poison rods are inserted into the guide thimbles of the fuel assembly.
The device according to U.S. Pat. No. 5,268,948 may include a sleeve fixed to the guide thimble and coming to bear under the adaptor plate of the nozzle, and two components which are also mounted on the end of the guide thimble so that they can move in rotation relative to one another about the axis of the guide thimble. One of these components is a ring rotatably mounted on the guide thimble and including two radially projecting parts. The adaptor plate of the nozzle is traversed by an opening of constant cross-section, the shape of the opening corresponding to the shape of the transverse section of the ring. The ring is mounted on the end of the guide thimble projecting out of the opening of the plate, and a second component fixed to the end of the guide thimble is provided in order to immobilize the ring in the position in which the guide thimble is locked or in the position in which it is unlocked. The device for fixing the guide thimble is therefore, in the main, situated outside the nozzle above the adaptor plate.